Comparative study of Co3O4(111), CoFe2O4(111), and Fe3O4(111) thin film electrocatalysts for the oxygen evolution reaction

Water electrolysis to produce ‘green H2’ with renewable energy is a promising option for the upcoming green economy. However, the slow and complex oxygen evolution reaction at the anode limits the efficiency. Co3O4 with added iron is a capable catalyst for this reaction, but the role of iron is presently unclear. To investigate this topic, we compare epitaxial Co3O4(111), CoFe2O4(111), and Fe3O4(111) thin film model electrocatalysts, combining quasi in-situ preparation and characterization in ultra-high vacuum with electrochemistry experiments. The well-defined composition and structure of the thin epitaxial films permits the obtention of quantitatively comparable results. CoFe2O4(111) is found to be up to about four times more active than Co3O4(111) and about nine times more than Fe3O4(111), with the activity depending acutely on the Co/Fe concentration ratio. Under reaction conditions, all three oxides are covered by oxyhydroxide. For CoFe2O4(111), the oxyhydroxide’s Fe/Co concentration ratio is stabilized by partial iron dissolution.

-The double Randles circuit used to model the impedance spectra, incorporating a serial resistor, R1, and two Randles circuits R2/CPE2 and R3/CPE3. CPE=constant phase element. R=resistor.

Specific capacitance measurements
Potentiostatic electrochemical impedance spectroscopy (PEIS) was performed on our samples at various stages of the experiment, as outlined in the protocol in the main text. The frequency range was 100 mHz to 500 kHz, with 10 points per decade in logarithmic spacing and an amplitude of 10 mV. The spectra were fitted using a double Randles equivalent circuit with one of the circuits representing the electrolyte-oxide interface and the remaining elements modelling the electrolyte, cabling resistances and electrical resistances within the sample holder. The double-layer capacitance was assumed to be equal to the pseudo-capacitance of the constant phase element of the oxide-electrolyte interface. Values for the various samples are listed in Table S1. Figure S2 -Fe content relative to the total metal content in a Co1+δFe2-δO4(111), δ = 0, thin film as determined from Fe 2p and Co 2p peak XPS peak intensities at different photoelectron detection angles relative to the surface normal. These data were acquired on pristine as-prepared films. The error bars are standard deviations computed from several data sets. Figure S2 shows the change of the Fe 2p intensity (relative the sum of the Co 2p and Fe 2p intensities) as a function of the electron detection angle for the pristine as-prepared Co1+δFe2-δO4(111) samples. The increase of the Fe 2p intensity with increasing detection angle indicates that the concentration of Co at the surface is smaller than in the bulk, which is compatible with a mostly iron-terminated surface. In bulk Co1+δFe2-δO4, the tetrahedral sites are occupied by Fe 3+ ions, and XPS does not indicate a change in the Fe oxidation state for the surface layer. Structural data of the three oxide films are shown in Figure S3. The large scale STM images (panels a-c) reveal wide terraces (100 nm wide and larger) for Fe3O4(111) and somewhat smaller ones for Co1+δFe2-δO4(111) (20-60 nm) and Co3O4(111) (10-30 nm).
The atomic scale STM images ( Figure S3, d-f) exhibit a certain density of point and extended defects. On Co1+δFe2-δO4(111), triangular pits and islands can be found, with sizes of up to 4 nm across. Paul et al. interpreted similar small triangular islands on Fe3O4(111) as adsorbates such as water agglomerates resulting from water in the chamber's residual gas atmosphere 1 , which might apply also to the Co1+δFe2-δO4(111) case. The LEED patterns exhibit hexagonal structures as expected for the (111) surfaces of these oxides.
The lower panel of Figure S3 shows STM images from a Co1+δFe2-δO4(111) film that was annealed in 1×10 -5 mbar O2 at 1000 K. This produced a highly ordered film with larger terraces more than 100 nm across. Chains of triangular islands are visible across the terraces, which, as mentioned above, may be due to residual water in the UHV chamber.  Figure S4: Proposed surface structure of Co1+δFe2-δO4(111) based on STM images and surfacesensitive XPS spectra, assuming an "ideal" bulk-like termination. The surface is terminated with a tetrahedrally-coordinated layer of Fe 3+ ions (brown). The mixed-colour spheres represent the octahedrally-coordinated cation sites that are equally occupied by Fe 3+ and Co 2+ (blue) ions. Figure S5 -XPS spectra of the O 1s region of the Fe3O4(111) film after the OER experiment, comparing the region where OER was performed and an area of the sample that was outside of the area studied electrochemically. Figure S5 shows that the OH-related O 1s shoulder centered around 533 eV is largely due to processes resulting from the exposure of the sample to the electrolyte and the OER.  c) The data shown in Figure S6 demonstrate that XPS peaks from the substrate material (gold or platinum) are essentially absent after electrochemistry for all three oxide films, so that they do not affect the experimental results. units]
The initial LSV up to OER conditions has a similar slope and offset as that of the pure Fe3O4(111) film. A CA was performed to observe how the current density varied over time, starting at 1 mA/cm 2 ( Figure S8). It was found that the film initially behaved similar to the Fe3O4(111) film, with an initial increase and then a steady decline in activity to ~0.75 mA/cm 2 . However, after approximately 1 hour the current density began again to increase. This gradual increase continued for the following ~18 hours until it reached ~1.25 mA/cm 2 , like that of the most active Co1+δFe2-δO4(111) film after 2 hours. Apparently, Co was enriched in the oxyhydroxide layer and the Fe concentration was reduced via Fe dissolution. Surface-sensitive Fe 2p and Co 2p XPS data (electron detection angle 70°) indicate a decrease of the iron concentration from 78 % to ~50 %.  (1) (2) Figure S10 -Time-dependent chronoamperometry measurements of Co3O4 (111)  We assign occasional current jumps in the Fe3O4(111) and Co1+δFe2-δO4(111) CA data in Figure S10 to the formation of bubbles. No such features are observed in the Co3O4(111) curve.  Co1+δFe2-δO4(111) no additional peaks appeared over the course of the measurements. For the Fe3O4(111) film, however, a peak appears as a shoulder on the higher-wavenumber side of the main peak at 664 cm -1 . There is also a small amount of additional intensity observed at ~350 cm -1 .
These peaks are known for γ-Fe2O3, but not for α-Fe2O3 nor for FeOOH, confirming that the   Table S2 -Resistances (Ω) for the IR correction of the CVs in Figure 4. The numbers were obtained from PEIS data as explained in the methods section (main text).

Fe3O4(111) 92 88
Co3O4 (111)  84  87 CoFe2O4(111) 100 108 Table S3 -High-frequency cell resistances (HFR) in Ω for the CV plots in Figure 4. The numbers were obtained from PEIS data as explained in the methods section. We attribute the significant difference between the before-and after-OER values for Fe3O4(111) to the oxidation of the iron oxide.

Electrochemical cell
To ensure that all surfaces in contact with the sample and electrolyte were clean, the electrochemical cell, electrolyte and water bottles, tubing, and metal-free syringes were first immersed in a KMnO4 solution for 24 hours. This ensured that any carbon containing compounds were oxidised. Following this, everything was rinsed with ultrapure water before being immersed in a dilute piranha solution for several minutes. After again rinsing with ultrapure water, the components that can withstand elevated temperatures (i.e., all parts except the FKM seal) were boiled in ultrapure water. Finally, the non-metal parts were placed in a dilute HNO3 solution overnight to ensure that no metal contamination would be present. A final rinsing in ultrapure water was performed before the cell was inserted into the glass chamber filled with Ar gas where the electrochemical experiments took place.
We investigated the electrochemical (EC) performance of the thin films in an electrochemical cell attached to the UHV chamber, allowing sample transfer in situ without exposure to air. The sample was first transferred to a load-lock, which was vented using pure Ar gas, and from there to an attached glass chamber, also filled with Ar gas. Here, the polytetrafluoroethylene (PTFE) electrochemical cell was pressed against the sample, sealing a circular area of the surface with a diameter of 6 mm, see Figure S12. For the Co3O4(111) films, the sealing part of the cell was replaced with a fluorocarbon (FKM) piece to avoid damaging the Au (111)  prevent exposure of the electrolyte to air or foreign metals. Following electrochemistry, the sample was rinsed using ultrapure water (which also had Ar bubbled through it), before being reintroduced to the load-lock chamber. The load-lock was pumped down to UHV using a turbomolecular pump, before the sample was reintroduced to the UHV analysis chamber for "quasi in situ" postelectrochemistry analysis.

Ultra-high vacuum chamber
An UHV chamber with a base pressure of 4 × 10 −11 mbar was used for sample preparation and surface characterization with X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and scanning tunnelling microscopy (STM) at room temperature. The XPS setup comprised an X-ray source with Al and Mg anodes and a hemispherical analyser, all from Omicron GmbH, Germany. Unless stated otherwise, measurements were made at normal emission geometry (0° with respect to the surface normal) using Mg Kα radiation (1253.6 eV). For some measurements the surface sensitivity of XPS was enhanced by measuring at non-normal detection angles. The analyser was operated with a pass energy of 20 eV in constant analyser energy mode.
For binding energy calibration, the Fermi edge and the 4f peaks of a Au(111) crystal were used. A Shirley background was subtracted from the spectra, and peaks were fitted using the pseudo-Voigt functions of the CASA XPS software. 4 An Ar + ion gun was employed for sample cleaning. The sample could be heated with the sample set to a positive voltage of up to 1000 V either via thermal radiation or via electron bombardment using a tungsten filament mounted behind the sample. A K-type thermocouple spot-welded to the side of the substrate was employed for sample temperature measurement.

Layer thickness estimation from the damping of LEED spots.
The procedure is based on the damping of the LEED spots by the presence of the layer. For a kinetic energy of 160 eV, the IMFP λ is ~0.6 nm for Co3O4. It may be not much different for the hydroxide and the oxyhydroxide. The LEED spot damping may be calculated as D = e − H λ with H being the electron travel distance in the layer. H (and the layer thickness) can be computed from the experimentally observed LEED spot intensity damping. This is rather inaccurate since the arrangement of the oxide surface atoms may be affected by the presence of the layer and the IMFP value does not account for diffuse elastic scattering which will additionally weaken the LEED spots.

Some details of the OER activity calculations
The OER activity numbers are computed for 1 nm 2 electrochemical surface area. We choose to report activity per unit area rather than turnover frequency because, with the transformation of the surface to an oxyhydroxide layer, we cannot be certain of the number of active sites per unit cell. We assume that all of the current is used for O2 production, i.e., we assume that there is no pseudocapacitive contribution, which might be acceptable in view of the slow voltage ramp speed.
The ECSAs were determined from large area (500x500 nm) STM scans as described in the main text.

Computation of sampling depths for XPS
The sampling depth depends on the electron exit angle θ. If the angle is given relative to the surface normal, then the sampling depth D can be computed as D = λ × cos(θ), where λ is the inelastic mean free path length of electrons, IMFP. The IMFPs were computed with the IMFP TPP2M program 5 for Co3O4. For Fe 2p, λ = 12.0 Å and 4.1 Å at electron detection angles of 0° and 70°, respectively. For Co 2p, the λ values are 10.9 Å and 3.7 Å. 63% of the XPS intensity stems from a layer with the thickness D, 86% from a 2D thick layer.